Climate change-driven dual heatwave-driven drought-wet extremes (DHDWEs)—characterized by abrupt drought-flood transitions under heatwaves—pose growing threats to urban sustainability. Analyzing 334 Chinese cities (1980–2023), we identified newly rising DHDWE frequency (0.34 ± 0.09 events yr−1) and intensity (0.08 ± 0.04 units yr−1), with distinct north-south spatial divergence. We introduced a novel “time tightness” metric to indicate the relative time available for recovery or response between the two extreme phases of a DHDWE, revealing a dangerous downward trend. Panel model analysis shows that DHDWEs cause nonlinear economic losses, with a potential early-warning threshold emerging when time tightness falls below 0.33. Projections under SSP585 suggest DHDWE frequency will surge by 270 ± 120% by 2100, further compressing recovery windows. We therefore propose a Threshold-Alert, Window-Action framework that uses empirical timing thresholds to guide risk prioritization and earlier preparedness, supporting proactive adaptation for cities facing compressed recovery windows and rising economic risk. ... Read more

Green infrastructure measures such as street tree planting and ground surface greening effectively mitigate urban heat. As heat stress intensifies, identifying effective spatial arrangements of these interventions becomes increasingly important. To provide actionable guidance for real-world projects, practical design and resource constraints must also be considered. However, studies that systematically balance cooling effectiveness with practical applicability remain limited. This study develops design guidelines on the size, amount, and spatial distribution of street trees and grass in urban neighbourhoods, focusing on applicability in urban design practice. Four neighbourhood typologies in temperate-climate European cities were used as case examples, for which multiple greening scenarios were generated. A research-through-design approach was adopted: focus group workshops with practitioners evaluated the scenarios in terms of visual experience, functionality, cost, and maintenance; then the refined designs were assessed using ENVI-met simulations. The refined scenarios reduced mean PET by 1.07–7.89 °C compared with the no-greening reference case, with the strongest reductions in wide-street typologies. However, greater green coverage did not necessarily lead to stronger cooling: in a narrow-street typology with diagonal street orientations, the tree-only scenario reduced mean PET by 1.79 °C, while the tree–grass scenario with higher tree and grass coverage reduced mean PET by 1.33 °C. For each typology, two recommended scenarios are identified: one maximising cooling capacity and one prioritising cooling efficiency under space and resource constraints. By translating thermally ranked scenarios into practitioner-refined greening guidelines, this study provides design support for neighbourhood-scale, morphology-specific heat mitigation through the strategic spatial arrangement of street trees and grass. ... Read more

Global analyses of urban greenspace have traditionally highlighted a North-South disparity, yet a critical gap remains in understanding how climatic and socioeconomic factors differentially drive greenspace exposure inequality across these regions. To address this, we introduce a novel Greenspace Exposure Inequity Index (GEII) and apply it to 1,370 cities worldwide. Our analysis reveals a more complex picture than the conventional divide: while cities in the Global South generally exhibit slightly higher inequality, those in arid regions of the Global North display greater inequity. Through a machine learning approach, we identify distinct primary drivers—the Human Development Index (HDI) dominates inequality in Global Northern cities, whereas GDP, greenspace coverage ratio (GCR), and total annual precipitation (TAP) are pivotal in the Global South. Crucially, we detect statistically significant nonlinear thresholds that act as tipping points in these relationships. Specifically, thresholds for GDP (natural-logarithm scale: 23.851) and TAP (1017.158 mm) are identified in the Global South, and for GCR (0.55) in the Global North. The identification of these thresholds provides actionable insights for urban planning: prioritizing human-centered development and equitable greening in the Global North, and optimizing greenspace planning coupled with water resource management in the Global South, to mitigate greenspace exposure inequality effectively. ... Read more

In high-density cities with hot-humid climates, Urban Riverside Greenways (URGs) serve as critical cooling corridors, vital for mitigating urban heat island (UHI) effects and improving pedestrian thermal comfort. However, the synergistic cooling mechanisms between water bodies and vegetation, and how they interact with airflow within URGs, remain insufficiently quantified. This study presents a parametric analysis integrating validated ENVI-met simulations and field measurements, based on a case study in Shanghai. Through 75 factorial scenarios, it investigates the synergistic effects of prevailing wind direction, walkway position, and tree canopy characteristics —specifically Leaf Area Density (LAD) and foliage albedo—on pedestrian thermal comfort within URGs. Critically, this study dissects the trade-offs between water-based cooling (driven by high specific heat capacity and evaporative latent heat exchange) and vegetative cooling (driven by radiative shading and transpiration), and reveals how airflow redistributes these effects across the URG profile. The results show that: (1) Convective heat transfer (governed by prevailing wind direction) is the dominant factor regulating air temperature ( T a ), whereas radiative shading (determined by tree canopy characteristics) is the primary driver for Physiological Equivalent Temperature (PET). (2) The spatial distribution pattern of thermal comfort exhibits a tipping point governed by the equivalence in cooling capability between vegetation and waterbody. In the context of this study, this threshold occurred at LAD = 1.0 and foliage albedo =0.2 under full canopy cover. (3) Optimal walkway position is determined by the airflow-driven redistribution of water-based and vegetative cooling: upwind waterbodies favor water-adjacent locations. Conversely, downwind scenarios trigger a trade-off: street-side placement is superior under dense canopies (shading-dominant), whereas water-side remains optimal under sparse canopies. By synthesizing these micro-physical interactions, this study proposes a climate-adaptive decision framework, including decision trees and a parametric tree species selection matrix, providing actionable design strategies for thermally resilient URGs in hot-humid high-density cities. ... Read more

Street trees and grass are important elements for cooling cities, yet where and how to distribute them at the neighbourhood scale is still unclear. This study aims to identify effective street tree and grass design scenarios to maximise cooling in European neighbourhoods with temperate climate. 32 design scenarios were developed by combining urban design parameters of vegetation type, coverage, and spatial distribution in four neighbourhood typologies. The microclimate effects of these scenarios were then simulated using the ENVI-met model. To evaluate their cooling effects, three indices were applied: PET reduction, percentage change in thermal sensation class, and cooling efficiency. Results reveal that even fully covering streets with grass has a marginal thermal impact in reducing the mean Physiological Equivalent Temperature (PET) by up to 1.1 °C across the neighbourhood, while street trees lower PET by up to 8.7 °C. Neighbourhoods with wide radial streets have higher initial PET values and benefit more from green interventions. Strategically placing two rows of large trees on main streets is more effective for cooling than a single row on both main and secondary streets on the high-radiation side. Neighbourhood-specific practical recommendations for strategically implementing street trees and grass are provided to improve urban cooling. ... Read more